1. The Field of the Invention
The present invention relates to vertical cavity surface emitting lasers. More particularly, the present invention relates to vertical cavity surface emitting lasers that generate high power in a single mode using photonic crystals.
2. Background and Relevant Art
One of the light sources used in optical communication systems is a vertical cavity surface emitting laser (VCSEL). VCSELs are popular in part because they can be manufactured in large quantities due to their relatively small size and can be tested in wafer form. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well to optical fibers.
In a VCSEL, the light or optical signal being amplified resonates in a direction that is perpendicular to the pn-junction. The cavity or active region of a VCSEL is thus relatively short and a photon has a small chance of stimulating the emission of an additional photon with a single pass through the active region. To increase the likelihood of stimulating the emission of photons, VCSELs require highly efficient mirror systems such that a photon can make multiple passes through the active region. The reflectivity requirement of VCSELs cannot easily be achieved with metallic mirrors.
VCSELs thus employ Distributed Bragg Reflector (DBR) layers as mirrors. The semiconductor materials or dielectric materials used in DBR layers are grown or formed such that each layer has a refractive index that is different from the refractive index of adjoining layers. The junctions between the DBR layers that are grown in this fashion cause light to be reflected. The amount of light reflected, however, by a single junction is relatively small and is often dependent on the variance between the relative refractive indices of the adjoining materials. For this reason, a relatively large number of DBR layers are formed in a VCSEL in order to achieve high reflectivity. VCSELs, for example, often have on the order of 50 to 100 DBR layers in order to achieve sufficient reflectivity.
Even though forming a large number of DBR layers can be a difficult task, VCSELs are attractive for various low power and/or multi-transverse mode applications because they are easy to test, have a good beam profile and typically have low power consumption. These attributes also make VCSELs unattractive for other applications, such as those that require more power in a single mode. For example, high power pump lasers are required for erbium doped fiber amplifiers. The power supplied by the pump lasers should be delivered in a single mode in order to avoid excessive noise. Current VCSELs are not suitable for this application because single mode VCSELs do not produce sufficient power. Those VCSELs that do produce sufficient power introduce excessive noise because more than one mode is generated. As a result, higher cost edge emitting lasers are often used for these applications.
A single mode high power VCSEL is difficult to produce for various reasons. The wavelengths reflected by the DBR mirrors depend on the composition and thickness of the DBR layers. The resonance wavelengths of the cavity depend on the composition and thickness of the cavity and of the DBR layers. Growing VCSEL wafers is difficult because the thickness, composition, and doping requirements must all be monitored at the same time for, potentially, hundreds of layers. Even when the growth and fabrication issues are overcome, high power VCSELs generate light output with high order transverse modes. A single mode high power VCSEL would overcome these and other problems.
These and other limitations are overcome by the present invention which relates to single mode high power vertical cavity surface emitting lasers. Vertical cavity surface emitting lasers include mirror layers or regions that reflect photons through an active region repeatedly. This is necessary in order to stimulate the emission of additional photons, which ultimately results in the laser output. Distributed Bragg Reflector (DBR) layers are used as the mirrors in most VCSELs and the present invention is a VCSEL where at least one of the mirror layers of a VCSEL is formed from a photonic crystal or from a combination of photonic crystals and DBR layers.
A photonic crystal or layer is produced by creating cavities or apertures in a material such as a semiconductor material or a dielectric material. The cavities in the photonic crystal form a periodic cavity structure. In one embodiment, the periodicity of the cavity structure is on the order of the light wavelengths in the material (typically a few hundred nanometers). The periodic nature of the cavity structure is similar to the atomic lattice structure of various materials, but on a larger scale. The periodic cavity structure of a photonic crystal is not confined to or limited by the atomic lattice structure of the material and can be formed as required using various configurations and shapes. Because the photonic crystals used in the present invention are usually planar in nature, the periodic cavity structure is usually two dimensional, although a three dimensional cavity structure is contemplated by the present invention.
When a VCSEL is fabricated in accordance with the present invention, a photonic crystal is included in at least one of the mirror layers of the VCSEL. In other words, the photonic crystal can either replace the DBR layers or can be formed in addition to the DBR layers. In the latter case, the number of DBR layers can be reduced. The photonic crystal has high reflectivity such that photons are reflected by the photonic crystal back through the active region, which results in stimulated emission of photons at the corresponding wavelength of the incident photon.
The optical properties of the photonic crystal include sensitive to both the wavelength and the incident angle of the photons. Including a photonic crystal in the structure of the VCSEL enables a single mode to be generated because other wavelengths of light are not reflected and do not have appreciable gain. The wavelength-dependent reflectivity of the photonic crystal enables the VCSEL to generate a single output mode. The power of the VCSEL can be increased by enlarging the aperture through which light is emitted. Enlarging the aperture does not increase the number of modes generated by the VCSEL because the photonic crystal only has high reflectivity for a particular mode.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.